This invention relates in general to techniques for generating on-chip clock signals in an integrated circuit ("IC") and in particular, to a technique and on-chip circuit for automatically detecting at power up whether an off-chip crystal oscillator or an on-chip RC oscillator has been selected to generate a system clock signal for other on-chip circuitry, and then providing the system clock signal generated from the selected oscillator to the other on-chip circuitry.
U.S. Pat. No. 5,093,633, entitled "Externally Trimmed Integrated-Circuit RC Oscillator," and incorporated herein by reference, discloses a technique invented by the inventor of the present invention, which selects through a mask programmable option in the manufacturing process, either an off-chip crystal oscillator or an on-chip RC oscillator option for generating an on-chip system clock signal. Using the technique disclosed therein, once the selected option is mask programmed onto the IC at the IC manufacturer's factory, the selected option becomes permanent.